esc: export color_set to its own function
have a config to choose whether to set TERM to xterm-256color
add -2/--256color option to set TERM to xterm-256color

Man page was modified accordingly.
compat: fix backspace

According to
http://invisible-island.net/ncurses/terminfo.ti.html#tic-xterm-basic ,
xterm sends ^H == \b == 0x08.
set correct mode with fcntl
screw terminfo: have a config to decide whether erase is del or backspace

One day I'll have to dig into the terminfo/termcap mess :(
Maybe get the O'Reilly book about it…